Natural gas (NG) having methane as major component has already been known as potential fuel for vehicular application. Vehicles running on compressed natural gas (CNG) are on the roads. However, there are pros and cons associated with use of CNG. The alternate lies in the use of adsorbed natural gas (ANG). Considering the potential of ANG, the Department of Energy (DoE) in the United States has set targets as 180 v(STP)/v, at 3.5 MPa and 298 K [STP is standard temperature (298 K) and pressure (0.1 MPa)], for methane storage in vehicular application. Research on developing efficient materials and enhancing the capacity of known materials such as porous silicates, carbons, and MOFs have been pursued as means for methane storage. Although each of the prior art work to some extent, more efficient storage materials are necessary to cope with the DoE targets. The highest methane storage capacity obtained in activated carbons is ca. 200 v/v [Wegrzyn, J., and Gurevich, M., “Adsorbent storage of natural gas”, Appl. Energy, 55, 71-83 (1996)], although significant efforts were made on processing activated carbons.
Metal organic-frameworks (MOFs) are a new class of nanoporous materials that have potential applications in separation processes, catalysis and gas storage. MOFs are synthesized using organic linker molecules and metal clusters that self-assemble to form materials with well defined pores, high surface areas, and desired chemical functionalities. Because of these attractive properties, MOFs are promising candidates for CO2 capture, as well as methane and hydrogen storage.
A variety of MOFs have been screened for methane storage [Wang, S., “Comparative molecular simulation study of methane adsorption in metal-organic frameworks”, Energy & Fuels, 21, 953-956 (2007); Noro, S., Kitagawa, S., Kondo, M., Seki, K., “A new, methane adsorbent, porous coordination polymer [{CuSiF6(4,4′-bipyridine)2}n]”, Angew. Chem. Int. Ed., 39, 2081-2084 (2000); Kondo, M., Shimamura, M., Noro, S. I., Minakoshi, S., Asami, A., Seki, K., Kitagawa, S., “Microporous materials constructed from the interpenetrated coordination networks. Structures and methane adsorption properties”, Chem. Mater., 12, 1288-1299 (2000); Bourrelly, S., Llewellyn, P. L., Serre, C., Millange, F., Loiseau, T., Ferey, G., “Different adsorption behaviors of methane and carbon dioxide in the isotopic nanoporous metal terephthalate MIL-53 and MIL-47”, J. Am. Chem. Soc., 127, 13519-13521 (2005); Düren, T., Sarkisov, L., Yaghi, O. M., Snurr, R. Q., “Design of new materials for methane storage”, Langmuir, 20, 2683-2689 (2004); Ma, S., Sun, D., Simmons, J. M., Collier, C. D., Yuan, D., Zhou, H. C., “Metal-organic framework from an anthracene derivative containing nanoscopic cages exhibiting high methane uptake”, J. Am. Chem. Soc., 130, 1012-1016 (2008).], but only a few can reach the DoE target. For example, Düren et al. [“Düren, T., Sarkisov, L., Yaghi, O. M., Snurr, R. Q. Design of new materials for methane storage”, Langmuir, 20, 2683-2689 (2004)] proposed a theoretical MOF (IRMOF-993) with a methane adsorption capacity of 181 v(STP)/v. Ma et al. [Ma, S., Sun, D., Simmons, J. M., Collier, C. D., Yuan, D., Zhou, H. C., “Metal-organic framework from an anthracene derivative containing nano-scopic cages exhibiting high methane uptake”, J. Am. Chem. Soc., 130, 1012-1016 (2008)] synthesized a MOF named PCN-14 that gave the highest methane adsorption capacity of 230 v(STP)/v so far. However, they used crystal density rather than packed density in arriving at this value.
Catenated MOFs are composed of two mutually catenated frameworks that generate additional pores with various sizes. The catenation structure strengthens the gas affinity for the material by an entrapment mechanism that improves the gas adsorption capacity and separation. Thus, catenation appeared to be a useful strategy for designing new MOFs as efficient methane storage materials. Based on this consideration, a systematic molecular simulation study is performed to investigate the effect of catenation on methane storage capacity to provide useful information for further MOFs development with improved methane storage capacity. [XUE Chunyu, ZHOU Zi'e, YANG Qingyuan and ZHONG Chongli, Enhanced Methane Adsorption in Catenated Metal-organic Frameworks: A Molecular Simulation Study, Chinese Journal of Chemical Engineering, 17(4) 580-584 (2009)]. This work also showed that catenated MOFs can meet the DoE target easily for methane storage, indicating that the creation of catenated frameworks is a promising strategy for developing MOF-based efficient methane storage materials in vehicular applications. However, it is still theoretical study and no report published till date to prove such strategy.
U.S. Pat. No. 7,196,210 (Omar M. Yaghi, et al., Mar. 27, 2007) describes isoreticular metal-organic frameworks, process for forming the same and systematic design of pore size and functionality therein, with application for gas storage. An inventive strategy based on reticulating metal ions and organic carboxylate links into extended networks has been advanced to a point that has allowed the design of porous structures in which pore size and functionality can be varied systematically. MOF-5, a prototype of a new class of porous materials and one that is constructed from octahedral Zn—O—C clusters and benzene links, was used to demonstrate that its 3-D porous system can be functionalized with the organic groups. Indeed, the data indicate that members of this series represent the first mono crystalline mesoporous organic/inorganic frameworks, and exhibit the highest capacity for methane storage (155 cm3/cm3 at 36 atm.) and the lowest densities (0.41 to 0.21 g/cm3) attained to date for any crystalline material at room temperature. The drawback associated with this material is its low densities which result in less amount of material in a fixed volume.
US 20100069234, (Richard R. Willis, John J. Low, Syed A. Faheem, Annabelle I. Benin, Randall Q. Snurr, and Ahmet Ozgur Yazaydin, describes the use of certain metal organic frameworks that have been treated with water or another metal titrant in the storage of carbon dioxide. The capacity of these frameworks is significantly increased through this treatment. The limitation of this invention is that the method is shown suitable for storage of carbon dioxide and it does not teach about methane storage.
In the present invention, it is shown that MOFs, specifically Cu-BTC, can be easily tuned to significantly enhance methane storage capacity simply by synthesizing the Cu-BTC in presence of selected type and appropriate amount of activated carbon there by filling the void space. This method for enhanced storage of methane may apply to certain other guest molecules and other MOFs also.
It is common practice to activate MOFs at about above 150° C. to remove the solvent and open up the void space for the adsorption of desired gas molecules. If the evacuation temperature is high enough, all guest molecules entrapped during the synthesis can be removed, including those that are coordinatively bound to framework metal atoms. Removing these coordinated solvent molecules leaves coordinatively-unsaturated, open-metal sites that have been shown to promote gas uptake, especially for H2 adsorption. Recently, Bae et al. [Youn-Sang Bae, Omar K. Farha, Alexander M. Spokoyny, Chad A. Mirkin, Joseph T. Hupp and Randall Q. Snurr, Chem. Commun., 2008, 4135-4131] showed that in a carborane-based MOF removal of coordinated dimethyl formamide increased CO2 and CH4 adsorption and led to high selectivity for CO2 over methane. The open-metal sites in MOFs are reminiscent of the extra-framework cations in zeolites, in that they are expected to create large electric fields and to readily bind polar molecules. Methane being non-polar molecule is adsorbed in the overlapping force field created between two walls of a pore.
Cu-BTC (also known as HKUST-1) is a well-studied MOF, first synthesized by Chui et al. [S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen, I. D. Williams, Science 283 (1999) 1148-1150]. The structure of Cu-BTC is composed of large central cavities (diameter 9.0 Å) surrounded by small pockets (diameter 5.0 Å), connected through triangular-shaped apertures of similar size. The Cu-BTC framework has paddlewheel type metal corners connected by benzene-1,3,5-tricarboxylate (BTC) linkers. Each metal corner has two copper atoms bonded to the oxygen of four BTC linkers. In the as-synthesized material, each copper atom is also coordinated to one water molecule. MOFs have been found to have the capacity to store methane readily and at high selectivity over other gases such as nitrogen. In research publications, several MOFs that have the capacity to store methane are described. However, the storage capacity is not matching to the DoE targets, and therefore, it is necessary to enhance the capacity for methane storage to make them commercially useful.
In the present invention, a process has been developed and described for enhancing the gas storage capacity of MOFs, and especially for methane on Cu-BTC, by using “void space filling method” that have been accomplished simply by in-situ addition of selected type and appropriate amount of activated carbon, as ‘void space filling agent’, during the synthesis of Cu-BTC, thereby forming composite materials, AC@MOF. The gas storage capacity of this AC@MOF composite material is significantly increased as compared to the MOF synthesized without activated carbon.